In the secondary battery manufacturing process, it is required to detect products that are short-circuited between electrodes or products that may become short-circuited in the future. The detection is performed with respect to an electrode body including a positive electrode, a negative electrode, and a separator provided between the plates thereof, before an electrolyte solution is injected into a battery case. As a method of such detection, a method of measuring the insulation resistance of the electrode body when a voltage is applied between the positive and negative electrodes is disclosed (see JP 8(1996)-45538 A). Further, a method of determining whether a product is non-defective or defective by applying between the positive and negative electrodes a voltage that causes no dielectric breakdown in non-defective products but causes dielectric breakdown in potentially short-circuited products, more specifically a voltage of 100-400 V, is disclosed (see JP 2000-195565 A).
In the inspection method disclosed in JP 8(1996)-45538 A, it is impossible to determine whether insulation is sufficient or not unless the insulation resistance after a certain period of elapsed time is measured. This is because the electrode body has a low resistance due to the flow of an inrush current immediately after the application of the voltage. Further, in this method, since the insulation resistance is merely measured, it may be impossible to detect an electrode body that may become short-circuited in the future in which, although the positive electrode and the negative electrode are not in contact with each other, they are in close proximity to each other due to a defect of the separator or the like.
In the inspection method disclosed in JP-2000-195565 A, air discharge is used (see paragraph [0009], for example). In air discharge in an atmospheric pressure environment, when the distance between the electrodes is 25 μm or more, a discharging start voltage decreases with decreasing distance. However, when the discharge distance is shorter than 25 μm, a withstand voltage increases conversely, and discharge is not started until a higher voltage is achieved (e.g., see The Institute of Electrical Engineers of Japan, “Revised New Edition of Discharge Handbook”, Ohmsha, Ltd., published on Feb. 15, 1974, page 134). On this account, in the case where a foreign object is present between the positive and negative electrodes, so that a substantial insulation distance between the plates is approximately 25 μm or less, it is difficult to detect potentially short-circuited products even though a short circuit is likely to occur after injection of an electrolyte solution.
In recent years, lithium ion secondary batteries use a separator with a thickness of 25 μm or less. The application of the inspection method disclosed in JP-2000-195565 A itself to such secondary batteries is not suitable.